The present invention relates to papermaking machines and methods of making paper, and more particularly relates to machines and methods for making textured soft paper, such as tissue.
A paper machine for the production of tissue paper is disclosed in U.S. Pat. No. 5,393,384, see particularly FIG. 6. The paper machine shown therein has a belt impermeable to water, which runs in a loop through an extended press nip formed by a shoe press roll and a counter roll. A press felt is conveyed directly to the press nip, where it is brought together with the impermeable belt and the paper web. The paper web is transferred from a forming fabric to the impermeable belt which is to carry the paper web on its under side up to the press nip and thence to the drying cylinder. The impermeable belt thus carries the paper web a relatively long distance after the paper web has been transferred from the forming fabric to the impermeable belt. There is therefore a risk of the paper web not adhering sufficiently strongly along the entire distance and thus becoming detached from the impermeable belt. According to the patent specification the adhesion force between the impermeable belt and the paper web is greater than that between the press felt and the paper web. The impermeable belt under discussion here is not compressible and has a smooth, web-carrying surface.
It is generally known that such a smooth, impermeable belt obtains a film of liquid on its smooth, web-carrying surface when belt, press felt and paper web pass together through a press nip and that, after the press nip, the paper web therefore adheres to the impermeable belt instead of to the press felt which does not have a smooth surface, when the press felt and the impermeable belt run away from each other. This situation is also utilized in U.S. Pat. No. 4,483,745. Since, however, both the impermeable belt and the drying cylinder in the paper machine according to U.S. Pat. No. 5,393,384 have smooth surfaces with which the paper web is intended to come into contact, there is considerable risk of the paper web continuing to adhere to the smooth surface of the impermeable belt after it has passed the nip at the drying cylinder instead of being transferred to the smooth surface of the drying cylinder as desired. Probably not even the application of large quantities of adhesive on the envelope surface of the drying cylinder would ensure adhesion of the paper web to the drying cylinder. U.S. Pat. No. 5,393,384 mentions nothing about texturing the paper web before the drying cylinder.
DE-195 48 747 discloses a paper machine for manufacturing creped tissue paper which is provided with a press comprising a shoe press roll, a counter roll and a suction roll, the counter roll forming a first press nip with the suction roll and a second extended press nip with the shoe press roll. A felt passes through the two press nips together with the paper web and then carries the paper web with it to a Yankee dryer, to which the paper web is transferred when the felt and the paper web pass around a transfer roll forming a non-compressing nip with the Yankee dryer. Suction zones are provided before and after the first press nip, the suction zone before the press nip being situated within the suction roll whereas the suction zone after the press nip is in a side loop in which the felt runs alone and joins the paper web again at the entry to the second press nip. One drawback with such a paper machine is that the paper web is exposed to re-wetting by the wet felt before it reaches the Yankee dryer. The paper machine has no impermeable belt, nor does the patent specification mention anything about texturing the paper web.
U.S. Pat. No. 5,298,124 discloses a compressible transfer belt for use in a paper or board making machine in order to eliminate open draws in the paper web and to easily release the paper web so that it can be transferred to a fabric or belt. The transfer belt carries the paper web through the press section, which comprises one or more press nips, and on to the drying section which comprises a plurality of drying cylinders and a belt passing in a loop around a transfer roll which forms a nip with the transfer-belt. Each press is also provided with a felt passing through its press nip and enclosing the paper web between it and the transfer belt. The impermeable transfer belt is also so designed that a liquid film formed in a press nip between the transfer belt and the paper web breaks up when the pressure on the transfer belt ceases after the press nip so that its release properties increase and the paper web can thus more easily be transferred to a fabric or another belt running in a loop. There is no suggestion or intimation in the patent specification that the transfer belt should be allowed to carry the paper web to a drying cylinder in a tissue machine. Nor is there any mention of texturing the paper web.
U.S. Pat. No. 5,298,124 offers an excellent description of the tasks a transfer belt cooperating with a press felt shall perform in a satisfactory manner, and also of the properties and design of such transfer belts which then were disclosed in patent specifications U.S. Pat. Nos. 4,483,745, 4,976,821, 4,500,588, 5,002,638, 4,529,643 and CA-A-1,188,556.
According to U.S. Pat. No. 5,298,124, for a transfer belt intended for cooperation with a press felt the critical tasks are a) to remove the paper web from the press felt without causing instability problems; b) to cooperate with the press felt in one or more press nips to ensure optimal dewatering and high quality of the paper web, and c) to transfer the paper web in a closed draw from a press in the press section to a paper web receiving fabric or belt in the following press or presses of the press section or to a pick-up fabric in the drying section.
As mentioned, the transfer belt for the press section of a paper machine disclosed in U.S. Pat. No. 5,298,124 has a web-contacting surface which is substantially impermeable to water and air and has a pressure-responsive microscale topography. Under influence of the pressure in a press nip in the press section, the transfer belt is compressed so that the microscale roughness of said surface is decreased, whereupon the surface becomes much smoother and allows the formation of a thin, continuous film of water thereon.
Paper machines for manufacturing soft paper with high bulk are known through a plurality of patent specifications. An imprinting fabric or felt is generally used which passes, together with the paper web formed, through a press nip in which the paper web is pressed into the imprinting fabric, thus acquiring a texture pattern on one side. Paper machines having such texturing fabrics and press nips are disclosed in U.S. Pat. Nos. 3,301,746, 3,537,954, 4,309,246, 4,533,437, 5,569,358, 5,591,305 and WO 91/16493. The drawback with the paper machines disclosed in these publications is that dewatering in the press nip is relatively low and the dry solids content of the paper web is therefore low when the paper is transferred to the drying cylinder. The production rate of the paper machine is thus relatively low.
U.S. Pat. No. 4,849,054 discloses a machine for manufacturing an imprinted fabric web with high bulk without the use of a press nip. A roll, e.g., a transfer roll or felt-carrying roll, forms a nip with an imprinting fabric at a transfer point for the web where the imprinting fabric passes around a suction tube with a slit opening facing the transfer point. The nip is so wide that the web is not compressed when it passes through. The suction effect from the suction tube via the narrow slit opening is sufficient to ensure that the web is not only transferred to the imprinting belt but is also shaped in compliance with the surface of the imprinting belt facing the web, this belt having a three-dimensional pattern. Prior to the transfer point the speed of the fabric web is greater than that of the imprinting fabric. The roll carrying the web to the non-compressing nip has a smooth surface and it is generally known that in practice considerable problems are entailed in transferring a fabric web from a smooth surface to a fabric, which fabric web has been pre-pressed to a dry solids content of 30-50%.
U.S. Pat. No. 5,411,636 discloses manufacture of soft paper where the paper web is formed on a forming fabric, pre-pressed in a double-felted press nip and transferred to a coarse-meshed fabric. When the paper web is carried by the coarse-meshed fabric it is subjected to a vacuum in a suction zone so that the paper web is sucked into the openings and depressions in the fabric and thereby acquires increased thickness and thus increased bulk. The coarse-meshed fabric then carries the paper web to the drying cylinder. The double-felted press nip ensures that the dry solids content of the paper web is relatively low, i.e., 25-30%. Since no dewatering can be performed in the nip at the drying cylinder, the dry solids content of the paper web upon transfer to the drying cylinder is correspondingly low. Furthermore, it is extremely difficult to transfer the paper web from the felt to the coarse-meshed fabric.
Accordingly, an improved paper machine and method of manufacturing textured soft paper would enable the manufacture of a textured fibrous web with high bulk and high dry solids content before the drying cylinder to enable a high production rate to be achieved at a reasonable cost. Further, it would be desirable to reliably transfer the textured fibrous web to the drying cylinder although the fibrous web is carried to the drying cylinder by an impermeable texturing belt.
The paper machine according to the invention is characterized in that
a) the substantially impermeable belt is a texturing belt including a back layer and a web-contacting layer having a multitude of uniformly distributed depressions with surface portions located between them to form an equivalent relief pattern in the fibrous web during its passage through the press nip;
b) the substantially impermeable texturing belt is arranged to run from the press to the drying cylinder in order to carry the textured fibrous web to said transfer nip;
c) the press felt is arranged to run in a direction away from the impermeable texturing belt at a point immediately after said press nip and before a water film formed in the press nip on the substantially impermeable texturing belt breaks up; and
d) a device for applying adhesive is arranged before said transfer nip to apply a continuous adhesive layer on the envelope surface of the drying cylinder and/or on the textured fibrous web.
The method according to the invention is characterized by
a) texturing the fibrous web by means of the substantially impermeable belt, which is a texturing belt including a carrier and a web-contacting layer having a multitude of uniformly distributed depressions with surface portions located between them to form an equivalent pattern in the fibrous web during its passage through said press nip;
b) running the substantially impermeable texturing belt from the press to the drying cylinder in order to carry the textured fibrous web to said transfer nip;
c) running the press felt in a direction away from the impermeable texturing belt at a point immediately after said press nip and before a water film formed in the press nip on the substantially impermeable texturing belt breaks up; and
d) applying a continuous layer of adhesive on the envelope surface of the drying cylinder and/or on the textured fibrous web with the aid of a device for applying adhesive at a point before said transfer nip.
According to the invention it has surprisingly been found that impermeability or substantial impermeability is an extremely favorable property in a texturing belt, that is included in the paper machine according to the invention if the impermeable texturing belt is also used to transport a pressed paper web to the transfer nip at a Yankee dryer in the drying section of the paper machine. The property allows steam which, as a result of heating the Yankee dryer is formed in the depressions or pits in the texturing pattern by the water present in the pits or depressions, to be pressurized, thus pressing the paper fibers also present in the pits or depressions as a result of the press effect in the press section, so that these in the Yankee dryer nip are pressed into the pits or depressions at the same time as the parts of the paper fiber web present between the raised parts of the texturing pattern and the Yankee dryer become thinner. The desired texturing effect and high bulk of the paper web is thus achieved.
The texturing effect and the productivity can be increased if the texturing belt or a layer of the texturing belt intended for contact with the paper web is also given the feature of reversible compressibility so that the texturing belt is compressed in the transfer nip at the Yankee dryer. When the texturing belt then leaves the transfer nip and resumes its uncompressed state, a vacuum is created, which contributes to the formation of steam, which in turn facilitates separation of the texturing belt and paper web after the transfer nip and also quicker drying of the paper web on the Yankee dryer, i.e., higher paper production capacity. The vacuum-forming effect increases the quicker the belt resumes its uncompressed state, i.e., the more resilient the reversible compressibility is.
The texturing effect of the texturing belt that is included in the paper machine according to the invention is, of course, selected taking into consideration the desired texture pattern in the paper to be manufactured. The texture pattern is regular across the texturing belt or, if the texture pattern in the paper web is to include a particularly prominent additional pattern, e.g., a picture, logotype, etc., it has a regular basic pattern of depressions or pits and raised portions, onto which pattern the additional pattern is superimposed. xe2x80x9cRegularxe2x80x9d does not necessarily imply that the pattern appears regularly in all directions of the texturing belt. For instance, if the paper is soft paper that is to be creped, a tighter dominant transverse pattern (across the machine direction) as compared with a longitudinal pattern of elevations and pits, will give an increased creping effect. Thus, the pattern can be used for altering the properties of the paper in a desired direction.
Taking into consideration the material in the texturing belt or its surface layer that is intended to come into contact with the paper web, the texturing pattern can be achieved in some manner, known per se, such as etching, calendering, laser processing or embossing.
The density of the texturing pattern can also be used to influence the effect of the drying of the paper web on the Yankee dryer. Fewer contact points between the Yankee dryer and the paper web thus results in reduced drying effect from the Yankee dryer but increased drying effect from the hot air hood around the Yankee dryer on the fluffier parts of the paper web located between the thinner contact points.